1. Technical Field
The subject matter described here generally relates to measuring and testing for rate of flow, and, more particularly, to nacelle-to-freestream compensation of wind speed measurements for wind turbines.
2. Related Art
A wind turbine is a machine for converting the kinetic energy in wind into mechanical energy. If that mechanical energy is used directly by machinery, such as to pump water or to grind wheat, then the wind turbine may be referred to as a windmill. Similarly, if the mechanical energy is further transformed into electrical energy, then the turbine may be referred to as a wind generator or wind power plant.
Wind turbines use one or more airfoils in the form of a “blade” to generate lift and capture momentum from moving air that is them imparted to a rotor. Each blade is typically secured at its “root” end, and then “spans” radially “outboard” to a free, “tip” end. The front, or “leading edge,” of the blade connects the forward-most points of the blade that first contact the air. The rear, or “trailing edge,” of the blade is where airflow that has been separated by the leading edge rejoins after passing over the suction and pressure surfaces of the blade. A “chord line” connects the leading and trailing edges of the blade in the direction of the typical airflow across the blade. The length of the chord line is simply the “chord.”
Wind turbines are typically categorized according to the vertical or horizontal axis about which the blades rotate. One so-called horizontal-axis wind generator is schematically illustrated in FIG. 1 and available from GE Energy of Atlanta, Ga. USA. This particular configuration for a wind turbine 2 includes a tower 4 supporting a drive train 6 with a rotor 8 that is covered by a protective enclosure referred to as a “nacelle.” The blades 10 are arranged at one end of the rotor 8, outside the nacelle, for driving a gearbox 12 connected to an electrical generator 14 at the other end of the drive train 6 inside the nacelle with a control system 16.
In order to prevent damage to the blades, the control system 16 is typically configured to automatically start the wind turbine at minimum wind speeds of about 8 to 16 miles per hour, and then stop the turbine at maximum wind speeds of about 55 miles per hour. In addition, the control system 16 may also be configured to manage various other aspects of wind turbine operation, such as power output, power curve measurement, nacelle yaw, and blade pitch, in response to wind speed and/or other control system inputs. These and/or other aspects of the control system 16 are typically implemented under two broad, and often overlapping, classes of control systems having many variations and combinations: logic or sequential control, and feedback or linear control. However, so-called fuzzy logic may also be used to combine some of the design simplicity of logic control systems with the utility of linear control systems, and vice versa.
Feedback control systems typically include a control loop, with sensors, control algorithms, and actuators, that is typically arranged so as to regulate an operating parameter variable at a setpoint or reference value. So-called “PID control” is a common type of feedback control system that may be applied to wind turbines. “Open-loop” control systems, on the other hand, are used to control wind turbine operation in pre-arranged ways that do not make use of feedback.
Modern control systems 16 for wind turbines 2 are commonly implemented with local and/or remote computers, often as part of a locally embedded control system and/or a wider, distributed control system. These computers are typically configured to emulate logic devices by making measurements of switch inputs, calculating a logic function from those measurements, and then sending the results to electronically-controlled switches. Although both logic and feedback control systems are implemented for wind turbines with programmable logic controllers, the control system 16 may also be implemented with other non-computerized technologies such as electrical or mechanical relays, vacuum tubes, electronic, hydraulic, and/or pneumatic systems, and even simple, periodic manual adjustments.
As noted above, inputs to the control system 16 typically include various wind characteristics such as wind speed and direction taken from an anemometer with a vane. As illustrated in FIG. 1, the anemometer 18 may be mounted on or near the nacelle of the wind turbine 2. For example, the illustrated cup-type anemometer consists of cups at the ends of arms, which rotate when the wind blows. However, other types of anemometers may also be used, including vane-type anemometers, pressure-tube anemometers, hot-wire anemometers, and sonic anemometers.
The “nacelle wind characteristic measurements” that are made using these instruments are subject to a variety of effects that cause inaccuracies, including the turbulent effect of the blades 10 rotating into and out of the fluid flow path of the anemometer 18 and/or vane on the nacelle. Consequently, as illustrated in FIG. 2, an additional meteorological, or “met” mast 20 is sometimes arranged a suitable distance D upwind of the wind turbine 2 for taking “freestream” or “free stream” wind speed and direction measurements which approximate the “true” wind speed and direction that would have been measured at the turbine location had the turbine not been present. In fact, such wind speed and direction measurements are particularly important for “Power performance measurements of electricity producing wind turbines” as described in International Electrotechnical Commission Standard “IEC 61400-12.”
U.S. Patent Publication No. 2007/0125165 (from application Ser. No. 11/295,275) discloses a technique for correcting measurement error in data produced by a nacelle-based anemometer and for determining free stream wind speed which uses the following empirically derived formula
      V          nacelle      ,      corrected        =                    (                              a            -                                          C                p                            3                                b                )            *              V                  nacelle          ,          measured                      +    c  where Cp is a power coefficient and a, b, and c represent values associated with aspects of the turbine and operation thereof. However, without the met mast 20, these and other conventional nacelle wind speed error correcting techniques fail to adequately account for the turbulent effects of the blades 10 rotating into and out of the fluid flow path of the anemometer 18 on the nacelle. The relationship between this measured wind speed on the turbine nacelle and the actual freestream wind speed is sometimes referred to as the “nacelle-to-freestream transfer function,” or simply the “transfer function.”